Process and composition for etching ferrous metal surfaces



United States Patent ice 3,479,293 PROCESS AND COMPOSITION FOR ETCHINGFERROUS METAL SURFACES Kenneth P. Bellinger, Rockville, and Le VerneVerzier, Cheshire, Conn., assignors to Conversion Chemical Corporation,Rockville, Conn., a corporation of Connecticut No Drawing. Filed Sept.26, 1966, Ser. No. 581,736 Int. Cl. C09k 3/00; C23f 1/00 U.S. Cl.25279.4 9 Claims ABSTRACT OF THE DISCLOSURE A dry powder compositionupon addition to water provides an acid bath for etching ferroussurfaces and consists essentially of urea nitrate, a water-solubleauxiliary dry acidic component and optionally a finely divided inertfiller. The acidic components may be organic or inorganic acids or theacid salts thereof but do not include reducing agents or film formingradicals.

The present invention relates to the chemical etching of metals, andmore particularly, to a method for etching ferrous surfaces and to apowder composition for preparing the bath therefor.

Methods and compositions for etching steels and various other metalsare, of course, old in the art. In many cases, however, the rates ofattack of the available etchants are not readily controlled. Moreover,it is often difficult to obtain a desirable degree of uniformity inetching. Generally, the etchant employed for ferrous surfaces is nitricacid and the liquid acid employed creates dangerous-and otherwiseundesirable Working conditions.

Accordingly, it is an object of the present invention to provide aprocess for etching ferrous metal surfaces at a controlled rate and in amanner that will give substantially uniformly etched surfaces.

It is also an object to provide a self-contained dry powder formulationwhich, upon addition to water, will produce a highly effective bath forthe chemical etching of ferrous surfaces and which is convenient andsafe to ship and handle, relatively economical in operation andprecontrolled in the optimum proportion of essential ingredients so asto provide a balanced bath composition.

Another object is to provide such a self-contained dry powderformulation with which the rate of attack upon the metal workpieces canbe readily varied by varying the amount of powder formulation added towater and/ or the temperature.

It has now been found that the foregoing and related objects can bereadily attained by use of a self-contained dry power compositioncomprising urea nitrate and dry, auxiliary acidic component. Thecomponents and their concentrations are carefully selected so that uponsolution of the prepared powder in a suitable quantity of water, a bathwill resut having a pH of about 0.2 to 2.0 which will etch ferroussurfaces substantially uniformly. In addition to urea nitrate and theauxiliary acidic composition, a powdering or anti-caking agent, andpossibly other additives, are beneficially included in the powder. Thebath formed by the dissolved composition may be used at ambient andelevated temperatures, i.e., about 60 to 150 Fahrenheit, with the timeof immersion necessary for etching the workpiece varying with theconcentration and temperature of the bath.

More particularly, the powder compositions of the present inventioncomprise 80.0 to 95.0 percent, and preferably 85.0 to 90.0 percent, byweight of urea nitrate. Accordingly, the dry, auxiliary acidic componentcomprises 5.0 to

0 20.0 percent by weight, and preferably 10.0 to 15.0 percent 7 byweight, of the composition.

31,479,293 Patented Nov. 18, 1969 Although the theory of operation isnot understood, it is believed that the urea nitrate provides acontrolled source of nitric acid for the bath and that the urea exertssome surface activity enhancing the etching activity. At higherconcentrations and elevated temperatures with certain alloys, the ureanitrate acts to substantially eliminate fuming. The auxiliary acidiccomponent contributes to the controlled etching activity and appears toimprove the surface appearance while facilitating shipment of thecomposition as a desirably dry composition which is readily soluble inwater. It is possible that the auxiliary acidic compounds tie up certaindeleterious ions which occur in the solution. It is thought that, forexample, ferrous ions which are produced as a natural consequence of theetching process are tied up by the sequesting of chelating action ofcertain of the compounds such as the gluconic acids.

To be suitable, the auxiliary acidic component not only must be acidic,in the sense of the Lewis acid-base theory, but it must, of course, alsobe dry and water-soluble. Moreover, it must not be an active reducingagent and should not include a film-forming radical (i.e. phosphate andchromate radical-containing compounds are unsuitable as the secondcomponent). Accordingly, various compounds may be employed as the acidiccompound such as the dry acids exemplified by boric, tartaric, maleic,citric, succinic and amalic acids. In addition to acids per se, numerousacidic salts can also be utilized exemplified by acidic tartrates andsulfates of the alkali metal, alkaline earth metals and ammonia. Thebisulfates are particularly advantageous in that they aid in the controlof pH in the solution. Other compounds which are particularly suitableare those which conform to the above criteria and are also chelating orsequestering agents. Exemplary of such compounds are gluconic acid andparticularly the delta-lactone thereof.

In addition to the urea nitrate and the dry acidic component, as wasmentioned above, other components may also be advantageously included inthe powder compositions. One such component is a powdering, oranti-caking, agent which is a finely divided inert filler such as afinely divided calcium silicate. When employed, such a powdering agentis advantageously included in an amount within the range of 0.5 to 5.0percent based on the weight of powder. The admixture of other componentssuch as surfactants, etc., which will occur to those skilled in the artis also within the scope of the present invention.

Thus, one of the preferred compositions in accordance with the presentinvention is composed of about 84.6 percent by weight of urea nitrate,about 8.7 percent by weight of boric acid, about 4.75 percent by weightof ammonium acetate and about 1.95 percent by weight of powdering agent.A second preferred composition comprises a quantity of urea nitrate andMicro-Cell E similar to the above, but includes the delta-lacone ofgluconic acid in lieu of the boric acid and ammonium acetate.

The etching solutions may be prepared in any suitable manner, one of theoutstanding advantages of the present invention being that the drycomposition need only be dissolved in water at the point of utilizationto produce the etchant. Hence, the desired components are premixed inprecise ratios at the point of manufacture where quality control is mosteffectively maintained, leaving only the most rundimentary measurementsto be carried out by the ultimate operator who need not be especiallyskilled to achieve desirable results. The bath may be readily replenished merely by addition of the powder formulation to again obtain ahigh degree of desirable etching activity.

Since the water is added by the ultimate user, the formulation is dryduring transit. This not only decreases the expense of transportation bykeeping the weight to a minimum, but it also facilitates packaging andhandling at all stages. The working solution will be prepared in anysuitable manner by dissolving the prepared powder in a suitable quantityof water. An amount of powder ranging from about 3.0 to 15.0 percent,and preferably from about 4.0 to 10.0 percent, by weight on the basis ofthe water has been found to yield etchants of the proper pH whichprovide high-quality surfaces. The quantity of powder composition whichcan be dissolved is dependent upon the temperature of the water so thatthe higher concentrations require elevated temperatures.

Although it is possible to employ baths of the present invention whichhave pH values near 2.0, it is preferred that the pH of the etchant beless than 1.0, and most desirably at about 0.6. Thus, while the operablerange-of pH values is from about 0.2 to 2.0, the preferred range is'from0.5 to 1.0. The pH of the solution is, of course, determined by thecomponents of the dry powder and the concentration of powder in thesolution.

As is indicated above, the present compositions and the processemploying same are suitable for the etching of ferrous surfaces. Theterm ferrous surfaces as used herein refers to surfaces of iron andferrous alloy articles and other metallic articles having a coating ofiron or ferrous alloy produced by any means. Thus, the various steelalloys are included. It is to be pointed out, however, that theso-called nitric acid-passivated metals such as stainless steels are notwell suited for etching in accordance with the present invention.Passivation is due, it is believed, to the generation of an oxide filmon the surface of the metal which interferes with the etching reaction.It is hypothesized that the reaction by which such an oxide film isgenerated in the presence of nitrate ion is catalyzed by chromium ornickel so that passivatable metals should not be utilized in baths ofthe present invention.

The instant process may be effectively carried out by a relativelysimple series of steps. Preliminary to the etching operation, it isusually desirable, and sometimes essential, that the surface of thearticle to be etched be freed of dirt, oil, grease and othercontaminants which might tend to interfer with the etching process orotherwise reduce the desirability of the surface of the productobtained. Methods for such cleaning operations are well known in theart. Having cleaned and, normally, dried the workpiece, it is thenimmersed in a solution previously prepared from a powder formulation asdiscussed above, the solution having a proper pH value and beingmaintained at a suitable temperature. Although it is not essential, itis usually desirable to provide agitation in the etching bath or to theworkpiece so that a uniformly etched product is obtained. After a shorttime, the article is removed from the etchant and subjected tosubsequent treatments which may include rinsing and neutralizationsteps. Solutions of compounds such as sodium hydroxide and sodiumcyanide are suitable for the neutralization step when it is employed,sodium cyanide being a particularly useful neutralizer since it alsoprovides some corrosion protection to the etched surface. Other salts,such as those of potassium, can be similarly utilized. As an additionalsubsequent step, it is frequently desirable to desmut the surface of thearticle such as with an anodic alkaline technique. This is particularlytrue when plating of the surface is contemplated. The composition of thesmut, which is in many cases believed to be a carbide, will depend uponthe components of a particular alloy. Oxidizing washes tend to stain theetched surfaces and should, therefore, be avoided.

Exemplary of the eflicacy of the present invention are the followingspecific examples wherein baths were prepared by addition to water ofdry powder compositions in accordance with the present invention.

Example 1 A dry powder formulation is prepared containing by weight 84.6percent urea nitrate, 8.7 percent boric acid, 4.75 percent ammoniumacetate, and 1.95 percent of finely divided calcium silicate.

Into 1.5 liter of water at ambient temperature is dissolved 90.0 gramsof the above powder formulation to produce an acid bath having a pHmeasured at 0.6. Hull cell panels (zinc-plated steel) 2% inches by 4inches were previously prepared by stripping the zinc with hydrochloricacid, rinsing, treating with sodium nitrite to prevent rusting,desmutting, drying and weighing them. Ten such panels were immersed inthe etchant which was at ambient temperature. After immersion in theagitated bath for about two minutes, the panels were withdrawn from thesolution, rinsed with water and weighed. All panels were observed to beuniformly etched with highly desirable dull-matte finishes.

A second group of ten panels was treated with the same solution in theabove-described manner. They were observed to have equally well-etchedsurfaces.

Third and fourth batches were also sequentially etched in the same bath.The quality of etch on these panels was observed to be somewhat lowerthan that of the panels in the first two replicates. Therefore, anadditional charge of 90.0 grams of powder was added to the bath.

Three replicates of ten panels each were subjected to etching in thethus-rejuvenated bath under conditions similar to those which wereemployed with previous replicates. The first group of panels subjectedto the rejuvenated bath had surfaces which were equally as well etchedas the original two batches of panels. Thereafter, the quality of etchbegan to diminish in the two subsequent replicates.

An additional charge of 90.0 grams was accordingly added, followed bytwo replicates of ten panels each. Thereafter, the bath was againrejuvenated with 90.0 grams of powder and three replicates of panelswere etched. The solution was rejuvenated twice more with 9.0 gramcharges, the solubility thereof in the solution at ambient temperaturebeing exceeded upon the addition of the final charge.

In all cases of the first replicate after rejuvenation of the bath, thequality of the etched surfaces obtained was good. The data obtained fromthese tests are summarized in the table which follows, which indicatesthe average weight loss per panel as a result of each etching, thepoints of addition of rejuvenating charge and panel appearance for eachgroup of panels. In the table, a rating of A indicates good etching andappearance. Panels rated B exhibited somewhat less etching at the centerthan at the edges. Those rated C were poorly etched in the center andslightly etched near the edges. The D-rated panels were classified aspoorly etched.

TABLE Panel 1.5 liters of bath appearance l Solubility of solutionexceeded.

Example 2 A test similar to that of Example 1 was performed. In thistest, however, the dry powder was composed of 89.0 percent of ureanitrate, 1.0 percent of the powdering agent and 10.0 percent of thedelta-lactone of gluconic acid. The etched panels which were obtainedwith the thus-modified formulation exhibited surfaces more finely andeven more uniformly etched than those obtained with the formulation ofExample 1.

Example 3 An etching composition was prepared as in Example 1, thepowder composition containing 89.0 percent urea nitrate, 1.0 percent ofpowdering agent and 10.0 percent of sodium bisulfate, all by weight.Panels obtained from an etching operation, carried out in substantialaccordance with the procedure of Example 1 exhibited etched surfacescomparable in quality to those obtained in that example.

The temperatures and times employed in the etching process of thepresent invention are generally interdependent and dependent upon theconcentrations of the active agents of the solution and also upon thedesired degree of etching. It is highly advantageous to operate thepresent process with the solution maintained at ambient temperature;i.e., 60 to 100 Fahrenheit. This is, of course, the most economical andconvenient temperature, and also one at which well-etched products areobtained. A more rapid reaction can generally be effected by raising thetemperature of the solution. This in itself will serve not to increasethe rate of reaction, but it will also increase the solubility of thepowdered components in water and thereby make possible a moreconcentrated and, hence, an even more active, solution. Surprisingly,the activity of baths containing the delta-lactone of gluconic acid Wasfound not to increase appreciably with elevated temperatures. Althoughhigher temperatures are feasible, temperatures of about 150 Fahrenheitare generally most advantageous; temperatures in the range of 60 to 100Fahrenheit are preferred.

The period of immersion can vary within wide limits, times of from aboutten seconds to about ten minutes being feasible and of one to fiveminutes being preferred. While an increased degree of control isobtained when the reaction is slow, by virtue of a dilute solution and/or a low temperature, it is often more economical and efficient tominimize the time necessary to carry out an etching cycle. In thepresent process, balancing of such considerations is a convenient andfacile matter.

Since the baths are somewhat corrosive, the tanks and handling equipmentshould be fabricated from acidresistant material, such as rigidpolyvinyl chloride or a glass fiber-reinforced resin, to avoidcontamination. The dry powder formulations are not in themselvescorrosive, but should be transported in suitable containers that willminimize the likelihood of inadvertent contact with water or excessiveamounts of moisture.

As is seen from Example 1, replenishment of the baths of the presentinvention is a simple matter which can be effected so as to avoid theneed for discarding of the used baths at frequent intervals. The limitof replenishment occurs only after multiple replenishment and buildup ofions in the bath makes further replenishment no longer feasible.

Thus, it can be seen that the present invention provides a process ofetching ferrous surfaces at a controlled rate and in a manner whichproduces substantially uniformly etched surfaces. The self-contained drypowders provided by the invention produce, upon addition to water, bathswhich chemically etch ferrous surfaces in a highly effective manner. Thepowders are convenient and safe to ship and handle and are economical inoperation. Their nature is such that the optimum proportions ofessential ingredients of the baths are precontrolled and the rate ofattack of metal workpieces is readily varied by varying the amount ofthe powder formulations added to the water.

Having thus described the invention, we claim:

1. A dry powder composition for addition to water to provide an acidbath for etching ferrous surfaces consisting essentially of 80.0 to 95.0percent by weight of urea nitrate, 20.0 to 5.0 percent by weight of anauxiliary watersoluble dry acidic component selected from the groupconsisting of organic and inorganic acids and the acidic salts thereofand up to 5.0 percent of a finely divided inert filler, said dry powdercomposition providing a pH of about 0.2 to 2.0 when added to water in aquantity ranging from about 3.0 to 15.0 percent based on the weight ofwater, said acidic component excluding active reducing agents and filmforming radicals.

2. The dry powder composition in accordance with claim 1 wherein saidauxiliary acidic component consisting essentially of a mixture of boricacid and ammonium acetate.

3. The dry powder composition in accordance with claim 1 wherein saidauxiliary acidic component additionally includes a sequestering agent ofacidic nature.

4. The dry powder composition in accordance with claim 3 wherein saidsequestering agent is the delta-lactone of gluconic acid.

5. In an etching process, the steps comprising dissolving in water fromabout 3.0 to 15.0 percent by weight, based upon the water, of aself-contained dry powder composition consisting essentially of about80.0 to 95.0 percent by weight of urea nitrate, about 20.0 to 5.0percent by weight of an auxiliary water-soluble dry acidic componentselected from the group consisting of organic and inorganic acids andthe acidic salts thereof and up to 5.0 percent of a finely divided inertfiller to prepare a bath having a pH ranging from about 0.2 to 2.0, saidacidic component excluding active reducing agents and film formingradicals; and immersing a workpiece having a ferrous surface in saidbath maintained at a temperature from about 60 to 150 Fahrenheit for aperiod of time sufiicient to etch said ferrous surface substantiallyuniformly.

6. The process in accordance with claim 5 wherein said period of timeranges from about ten seconds to ten minutes.

7. The process in accordance with claim 5 wherein 4 to 10 parts byweight of said self-contained dry powder composition is dissolved in 100parts by weight of water to prepare a solution having a pH of about 0.5to 1.0, said self-contained dry powder composition consistingessentially of about to percent by weight of urea nitrate, 5 .0 to 14.5percent by weight of an auxiliary acid component consisting essentiallyof boric acid and ammonium acetate, and 0.5 to 5.0 percent by weight offinely divided inert filler, said solution being maintained at ambienttemperature and said period of time of immersion of said workpiece beingabout one to five minutes.

8. The process in accordance with claim 5 wherein 4 to 10 parts byweight of said self-contained dry powder composition is dissolved inparts by weight of water, said self-contained dry powder compositionconsisting essentially of 85 to 90 percent by weight of urea nitrate,0.5 to 5.0 percent by weight of a finely divided inert filler, and 5.0to 14.5 percent by weight of the delta-lactone of gluconic acid.

9. The process in accordance with claim 5 wherein said workpiece isremoved from said bath and rinsed in an aqueous cyanide bath.

References Cited UNITED STATES PATENTS 3,171,767 3/1965 Bellinger et al.252-79.4 XR

MAYER WEINBLATI, Primary Examiner US. Cl. X.R.

